Understanding Your Support System: The Design of a Stable Metal–Organic Framework/Polyazoamine Support for Biomass Conversion

Introducing oligomeric or polymeric
units into metal–organic
frameworks (MOFs) can result in composites that have significantly
improved properties when compared with the individual MOF or polymer
building blocks. With such synergy in mind, this work presents the
design of a novel MOF/polyazoamine support that is used to stabilize
Pd nanoparticles (NPs). The resulting composite catalyst, tested in
the reductive amination of levulinic acid, is found to have a markedly
improved lifetime when compared to just the MOF or polymer support
containing Pd. It is demonstrated, for the first time, that the lifetime
enhancement stems directly from the polymer, which plays a dual role:
(i) the oligomer stabilizes the MOF support through the elimination
of certain vibrational modes associated with the framework ligand
and likely pore-filling effects in the largest MOF pore and (ii) the
Lewis base functionality on the oligomer backbone binds to the surface
of the Pd NPs, thus, increasing their activity and inhibiting their
aggregation. Several complementary spectroscopic (IR, X-ray photoelectron
spectroscopy, Raman spectroscopy) and computational tools (pore space
and topology analysis, molecular mechanics, and density functional
theory simulations) are used to describe the nature of the MOF–oligomer
interaction and identify the most likely location of the polymer within
the MOF pore.

将寡聚或聚合单元引入金属有机框架（metal–organic frameworks, MOFs）中，可获得相较于单一MOF或聚合物构筑单元性能显著提升的复合材料。基于这一协同效应思路，本工作设计了一种新型MOF/聚偶氮胺载体，用于稳定钯纳米颗粒（Pd nanoparticles, NPs）。所制备的复合催化剂在乙酰丙酸还原胺化反应中进行测试，结果显示，相较于仅负载Pd的MOF或聚合物载体，其使用寿命显著延长。本研究首次证实，使用寿命的提升直接源于聚合物的双重作用：(i) 该寡聚物通过抑制框架配体相关的特定振动模式，以及在最大MOF孔道中发挥的潜在孔道填充效应，稳定了MOF载体；(ii) 寡聚物主链上的路易斯碱性官能团可结合Pd纳米颗粒表面，从而提升其催化活性并抑制其团聚。本研究采用多种互补的光谱表征手段（红外光谱（infrared spectroscopy, IR）、X射线光电子能谱（X-ray photoelectron spectroscopy, XPS）、拉曼光谱（Raman spectroscopy））与计算工具（孔道空间与拓扑分析、分子力学及密度泛函理论（density functional theory, DFT）模拟），阐明了MOF与寡聚物之间的相互作用本质，并确定了聚合物在MOF孔道中最可能的分布位置。